Maybe it's been suggested here before (if yes please delete) but maybe it would be a good idea to use the same navigation system as Google maps.
Double tap and drag finger up or down to zoom in and out respectively.
I think it would be a really nice idea if you could integrate the way Google maps zooms in and out.
That's double tap and hold and slide up or down to zoom in or out
I'm loving the app already as it is but maybe this would improve it!